Kibana version: 6.4.0

Elasticsearch version: 6.4.0

Server OS version: CentOS Linux release 7.4.1708 (Core) (Linux 4.14.12)

Browser version: Chrome 68.0.3440.106

Browser OS version: Windows 7 Enterprise x64

Original install method (e.g. download page, yum, from source, etc.): yum

Describe the bug:

Timeseries Visual Builder fails to list available index fields when a non-default index pattern name is specified on the "Panel Options" tab.

Steps to reproduce:

1. Open TSVB, do not change any settings. The default "Count" aggregation with the default index pattern is rendered correctly.
2. Switch to the "Panel Options" tab, change the index pattern name to a different one. The "Count" aggregation is working as it should with this new index pattern.
3. Switch back to the "Data" tab, select a different aggregation from the "Aggregation" drop-down list.
4. Try to select a field from the "Field" drop-down list. TSVB displays a "There aren’t any options available" message.

Screenshot:
https://i.imgur.com/GrZr5hi.png

Expected behavior:
TSVB displays a valid field list in the "Field" drop-down menu.

Any additional context:
This has been tested with a fresh Kibana 6.4.0 instance (no plugins, latest release tarball from elastic.co).
TSVB visualizations created in older versions of Kibana work without any issues.
Based on the request log from Kibana, it seems that TSVB doesn't re-request the field list when the index pattern changes. The initial request for the default index pattern is there (it can be seen on the screenshot above) but after that there are no such requests.